Coating Compositions for Security Elements and Holograms

ABSTRACT

The present invention relates to the use of coating compositions, comprising shaped transition metal, especially silver, particles and a binder, wherein the ratio of pigment to binder is preferably such that the resulting coating shows an angle dependent colour change, for the production of security elements and holograms. When the coating compositions of the present invention are used in coating a hologram the obtained products show a an angle dependent colour change (flip/flop effect), different colours in reflection and transmission, an extremely bright OVD image and extremely strong rainbow effect, high purity and contrast.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/224,789, filed on Aug. 1, 2016, which is a continuation ofU.S. patent application Ser. No. 13/511,582, filed on Jun. 27, 2012, nowU.S. Pat. No. 9,453,132, which is a 35 U.S.C. §371 national stage patentapplication of international patent application PCT/EP2010/067898, filedon Nov. 22, 2010, which claims priority to EP 09177328.3, filed on Nov.27, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

INCORPORATION-BY-REFERENCE OF THE MATERIAL ON THE COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the use of coating compositions,comprising shaped transition metal, especially silver, particles and abinder, wherein the ratio of pigment to binder is preferably such thatthe resulting coating shows an angle dependent colour change, for theproduction of security elements and holograms. When the coatingcompositions of the present invention are used in coating a hologram theobtained products show a an angle dependent colour change (flip/flopeffect), different colours in reflection and transmission, an extremelybright OVD image and extremely strong rainbow effect, high purity andcontrast.

Description of the Related Art Including Information Disclosed Under 37CFR 1.97 and 1.98

GB-A-1,465,908 (U.S. Pat. No. 4,116,710) relates to a method for thepreparation of particulate metal, such as aluminium, which methodcomprises depositing a metal coating on a substrate by vapor,electroless or sputter deposition and removing the metal coating fromthe substrate by dissolving the substrate in a solvent therefor, thethickness of the deposited coating being such that upon dissolution ofthe substrate the metal is released as a plurality of metal particles.The metal platelets obtained by the process have a thickness of from 20to 100 nm, especially 35 to 60 nm.

U.S. Pat. No. 4,321,087 discloses a continuous process for preparingfinely divided metal particles comprising the steps of: (a) applying arelease coating onto at least one side of a continuous carrier sheet inan amount of from 0.75 to 1.50 lbs. of said release coating per side ofsaid carrier sheet, (b) depositing in the form of a thin film, a metalselected from the group consisting of aluminum, chromium, copper, steel,silver and gold, in an amount of from 35 to 45 nm thickness directlyonto said release coating, (c) passing said carrier sheet with saidrelease coating and said film of said metal through a solvent forsolubilizing said release coating but which is non-reactive with saidmetal, (d) removing said film of said metal from said carrier sheet in aparticulate form to produce the metal particles substantially free ofsaid release coating, and collecting the metal particles in anon-reactive solvent which is non-reactive with said metal. (e)concentrating the metal particles, and (f) breaking the metal particlesinto pigment particles having a particle size diameter between about 25and 50 microns.

WO0024946 discloses a process for making flakes comprising: providing avapor deposition chamber; placing a transport device in the vapordeposition chamber; providing a release coat source and a vacuumdeposition source in the vacuum deposition chamber directed toward thetransport device, in which the deposition source deposits a layer offlake material; applying a vacuum to the chamber, and while the chamberis evacuated, applying-alternate layers of a release coat from therelease coat source and a vapor deposited flake layer from the vacuumdeposition source to the transport device in sequence to build up amulti-layer sandwich of alternating flake material layers andintervening release coat layers, the release coat layers comprising adissolvable material that forms a smooth continuous barrier layer andsupport surface on which the flake material layers can be formed, sothat removal of the sandwich from the evacuated chamber yields amulti-layer sandwich which can be easily separated into flakes of fineparticle size by subsequent treatment with a material that essentiallycompletely dissolves the intervening release coat layers to remove themfrom the flakes.

According to WO0024946 another process for making metal flakes is aprocess of Avery Dennison Corporation for making flakes sold under thedesignation Metalure®. In this process both sides of a polyester carrierare gravure coated with a solvent-based resin solution. The dried coatedweb is then transported to a metallizing facility where both sides ofthe coated sheet are metallized by a thin film of vapor depositedaluminum. The sheet with the thin metal film is then returned to thecoating facility where both sides of the aluminum are coated with asecond film of the solvent-based resin solution. The dried coated/metalsheet is then transported again to the metallizing facility to apply asecond film of vapor deposited aluminum to both sides of the sheet. Theresulting multi-layer sheet is then transported for further processingto a facility where the coatings are stripped from the carrier in asolvent such as acetone. The stripping operation breaks the continuouslayer into particles contained in a slurry. The solvent dissolves thepolymer out from between the metal layers in the slurry. The slurry isthen subjected to sonic treatment and centrifuging to remove the solventand the dissolved coating, leaving a cake of concentrated aluminumflakes approximately 65% solids. The cake is then let down in a suitablevehicle and further sized by homogenizing into flakes of controlled sizefor use in inks, paints, and coatings. Metal flakes produced by thisprocess for use in printable applications such as inks are characterizedby a particle size from about 4 to 12 microns and a thickness from about150 to about 250 angstroms.

WO020090613 as well as WO03046245 discloses a process for thepreparation of flakes having a high aspect ratio in which the flakeshave an average particle size from about 4 to about 12 microns and asingle layer thickness from about 5 to about 500 angstroms.

WO02/094945 relates to a method for the production of plane-parallelplatelets, comprising the steps:

a) vapour-deposition, at a pressure below atmospheric pressure, of aseparating agent onto a carrier to produce a separating agent layer,

b) vapour-deposition, at a pressure below atmospheric pressure, of atleast one product layer onto the separating agent layer, and

c) dissolution of the separating agent layer in a solvent and productionof a suspension in which the at least one product layer is present inthe form of plane-parallel platelets, in which method the separatingagent is selected from the group consisting of anthracene,anthraquinone, acetamidophenol, acetylsalicylic acid, camphoricanhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid,biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone,dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid,8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin,7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalicacid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid,naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassiumsalt, phenolphthalein, phenothiazine, saccharin and its salts,tetraphenylmethane, triphenylene, triphenylmethanol, and also mixturesof at least two of those substances. The planparall platelets obtainedby said process have typically a thickness of 30 to 500 nm and adiameter of from 5 to 50 μm.

WO06/021528 relates to a process for the production of plane-parallelplatelets, comprising the steps:

a) vapour-deposition of a separating agent onto a carrier to produce aseparating agent layer,

b) vapour-deposition of at least one product layer onto the separatingagent layer, and

c) dissolution of the separating agent layer in a solvent and productionof a suspension in which the at least one product layer is present inthe form of plane-parallel platelets, wherein

the separating agent is selected from the group consisting ofanthracene, anthraquinone, acetamidophenol, acetylsalicylic acid,camphoric anhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid,biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone,dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid,8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin,7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalicacid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid,naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassiumsalt, phenolphthalein, phenothiazine, saccharin and its salts,tetraphenylmethane, triphenylene, triphenylmethanol, and also mixturesof at least two of those substances, especially pentaerythritol(C(CH₂OH)₄), trimesic acid (=1,3,5 benzene tricarboxylic acid),DL-alanine, DL-valine, 2,6-diaminopurine, ascorbic acid,1,3,5-benzenetricarboxylic acid, o-acetylsalicyclic acid, diphenic acid,terephthalic acid, pyrogallol, cyanuric acid, hexamethyltetramine(urotropin), fumaric acid, and 4-acetylbenzoic acid and also mixtures ofat least two of those substances.

WO07/057328 discloses a process for the production of plane-parallelplatelets, comprising the steps:

a) deposition of a separating agent I, which is dissolvable in water,onto a carrier to produce a separating agent layer,

b) vapour-deposition of a separating agent II, which is not dissolvablein water, onto the separating agent layer of step a),

c) vapour-deposition of at least one product layer onto the separatingagent layer of step b), and

d) vapour-deposition of a separating agent II, which is not dissolvablein water, onto the product layer of step c),

e) dissolution of the separating agent layer of step a) in water andproduction of a suspension in which the at least one product layer ispresent in the form of plane-parallel platelets, the top surface and thebottom surface, but not the side surfaces of which are covered by theseparating agent II, and

f) dissolution of the separating agent layer of steps b) and d) in asolvent and production of a suspension in which the product, comprisingat least one layer, is present in the form of plane-parallel platelets.

The aluminium flakes described in WO06/021528 and WO07/057328 have anaverage diameter of at least 2 especially from 2 to 20 μm, moreespecially from 3 to 15 μm, and most preferred from 5 to 15 μm. Thethickness of the aluminium flakes is generally from 10 to 150 nm,especially from 10 to 100 nm, and more especially from 30 to 60 nm.

WO2005/051675 relates to a method for forming a (security) productcomprising the steps of:

-   a) providing a sheet of base material, said sheet having an upper    and lower surface;-   b) depositing a metallic ink on at least a portion of the sheet of    base material; and-   c) forming a diffraction grating on at least a portion of the    metallic ink, wherein the optical density of metallic ink when    deposited is in the range of 0.2 to 0.8.

The average pigment particle diameter is in the range 8-15 μm and thethickness of the pigment particles is in the range 10-50 nm, especially19-21 nm.

WO2005/049745 discloses a coating composition for use in coating adiffraction grating, comprising metal pigment particles and a binderwherein the ratio of pigment to binder is sufficiently high as to permitthe alignment of the pigment particles to the contours of thediffraction grating. Preferably, the thickness of the pigment particlesis less than 50 nm. More preferably, the thickness of pigment particleis less than 35 nm. More preferably still, the thickness of pigmentparticle is less than 20 nm. Even more preferably still, the thicknessof pigment particle is in the range 5-18 nm. In one embodiment ofWO2005/049745, the thickness of the pigment particles is in the range10-50 nm. In another embodiment, the thickness of pigment particle is inthe range 10-30 nm. In another embodiment, the average thickness ofpigment particle is 17 nm. In another embodiment, the average thicknessof pigment particle is 12.5 nm.

BRIEF SUMMARY OF THE INVENTION

The present invention provides coating compositions which can show anangle dependent colour change (variation of color as a function of theviewing angle; flip/flop effect) as well as different colours inreflection and transmission, and, when used in printing holograms,results in (security) products which can show an angle dependent colourchange (flip/flop effect), different colours in reflection andtransmission, an extremely bright OVD image and extremely strong rainboweffect, high purity and/or contrast.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The FIGURE is a Transmission Electron Micrograph (TEM) of theirregularly-shaped silver platelets obtained in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The coating compositions comprise shaped transition metal particles(pigment) having a longest dimension of edge length of from 15 nm to1000 nm, preferably from 15 nm to 600 nm and particularly from 20 nm to500 nm, and a thickness of from 2 nm to 100 nm, preferably from 2 to 40nm and particularly from 4 to 30 nm and a binder, wherein the ratio ofpigment to binder is such that the resulting coating shows an angledependent colour change, i.e. a variation of color as a function of theviewing angle.

The layer formed from the shaped transition metal particles exhibits“color shifting”, meaning that the layer exhibits a change in color asthe layer is viewed at different angles. Said “color shifting” is causedsolely by the shaped transition metal particles without the use ofadditional colorants.

The resulting coating shows an angle dependent colour change, if theratio of binder to shaped transition metal particles (pigment) is below10:1, especially below 5:1.

The transition metal is selected from the group consisting of Cu, Ag,Au, Zn, Cd, Ti, Cr, Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt. Thetransition metal is preferably Ag.

The shaped transition metal particles are typically in the form ofnanoplatelets, trigonal and hexagonal nanoprisms, nanostars (branchedparticles), nanocubes, nanocrescents, nanodisks, nanowires, nanorods,nanohexagons, nanospheroids, nanocylinders, pyramids, variousnanopolyhedra or hollow structures, or they are nanolens-shaped ornanocone-shaped, i.e. they differ from regular spheres.

The longest dimension of the edge length of the shaped transition metalparticles may vary of from about 15 nm to about 1000 nm, preferably from15 nm to 600 nm. The thickness of the shaped particles may vary of fromabout 2 to about 100 nm, preferably from 2 to 30 nm.

Preferably, the shaped transition metal particles are in the form ofplatelets having a longest dimension of edge length of from 15 nm to1000 nm, preferably from 15 nm to 600 nm and particularly from 20 nm to500 nm, and a thickness of from 2 nm to 100 nm, preferably from 2 to 40nm and particularly from 4 to 30 nm. More preferred, some of the shapedparticles are in the form of trigonal and/or hexagonal prisms.Especially, the shaped particles may be obtained as a mixture ofplatelets of different shapes in which hexagonal and/or triangularand/or truncated triangular prisms make up more than 20%, preferablymore than 30% and in particular more than 50% of the total number ofshaped transition metal particles. The shaped particles areadvantageously monocrystalline.

The aspect ratio (longest dimension of edge length/thickness is at least1.5, especially in the range of 1.5 to 300, very especially in the rangeof 1.5 to 125.

It is widely known to use in banknotes security elements in the form ofstrips or threads. Printing the inks (offset, gravure, flexo, ink jet,screen or intaglio inks) of the present invention directly on paper, orother substrates results in a coating showing an angle dependent colourchange. This direct printing could replace the security elements in theform of strips or threads used in banknotes, which are made from atransparent film provided with a continuous reflective metal layer,vacuum deposited aluminium on polyester film being the commonestexample.

The effects obtainable by the coating compositions of the presentinvention are described on basis of coating compositions, comprisingshaped silver particles, but are not limited thereto:

The colours in transmission and reflection are dependant on thelight-absorption spectrum of the coating and the colour in reflectionmay be complementary to the colour in transmission in the physicalsense. Generally, particles with higher aspect ratio and/or largerlinear dimensions provide blue colour in transmission and, depending onpigment to binder ratio, silver, gold, bronze, copper or violet colourin reflection. Particles with lower aspect ratio and/or smaller lineardimensions provide violet, magenta, pink, green or brown colour intransmission and, depending on pigment to binder ratio, various coloursin reflection. For example, a blue colour is provided with silverparticles obtained according to Examples 1 (aspect ratio of about 6-7)and 2 (aspect ratio above 10).

The coating compositions comprise the shaped transition metal particlesand a binder. The binder is a high-molecular-weight organic compoundconventionally used in coating compositions. The high molecular weightorganic material for the pigmenting of which the pigments or pigmentcompositions according to the invention may be used may be of natural orsynthetic origin. High molecular weight organic materials usually havemolecular weights of about from 10³ to 10⁸ g/mol or even more. They maybe, for example, natural resins, drying oils, rubber or casein, ornatural substances derived therefrom, such as chlorinated rubber,oil-modified alkyd resins, viscose, cellulose ethers or esters, such asethylcellulose, cellulose acetate, cellulose propionate, celluloseacetobutyrate or nitrocellulose, but especially totally syntheticorganic polymers (thermosetting plastics and thermoplastics), as areobtained by polymerisation, polycondensation or polyaddition. From theclass of the polymerisation resins there may be mentioned, especially,polyolefins, such as polyethylene, polypropylene or polyisobutylene, andalso substituted polyolefins, such as polymerisation products of vinylchloride, vinyl acetate, styrene, acrylonitrile, acrylic acid esters,methacrylic acid esters or butadiene, and also copolymerisation productsof the said monomers, such as especially ABS or EVA.

Advantageously, the coating composition further comprises a solvent.

The binder may comprise any one or more selected from the groupcomprising nitrocellulose, ethyl cellulose, cellulose acetate, celluloseacetate propionate (CAP), cellulose acetate butyrate (CAB), alcoholsoluble propionate (ASP), vinyl chloride, vinyl acetate copolymers,vinyl acetate, vinyl, acrylic, polyurethane, polyamide, rosin ester,hydrocarbon, aldehyde, ketone, urethane, polythyleneterephthalate,terpene phenol, polyolefin, silicone, cellulose, polyamide, polyesterand rosin ester resins.

An angle dependent colour change can be obtained at a pigment/binderratio of about 10:1 to about 1:10, whereas a pigment/binder ratioof >10:1 results in silver coloured coatings and a pigment/binder ratioof <1:10 results in the loss of visible reflection.

The coating composition is preferably a printing ink. The ink accordingto the present invention comprises, as in the case of an ordinaryprinting ink, the shaped transition metal particles, a binder, anauxiliary agent, and the like.

With respect to the binder resin, a thermoplastic resin may be used,examples of which include, polyethylene based polymers [polyethylene(PE), ethylenevinyl acetate copolymer (EVA), vinyl chloride-vinylacetate copolymer, vinyl alcohol-vinyl acetate copolymer, polypropylene(PP), vinyl based polymers [poly(vinyl chloride) (PVC), poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), poly(vinylidene chloride)(PVdC), poly(vinyl acetate) (PVAc), poly(vinyl formal) (PVF)],polystyrene based polymers [polystyrene (PS), styrene-acrylonitrilecopolymer (AS), acrylonitrile-butadiene-styrene copolymer (ABS)],acrylic based polymers [poly(methyl methacrylate) (PMMA), MMA-styrenecopolymer], polycarbonate (PC), celluloses [ethyl cellulose (EC),cellulose acetate (CA), propyl cellulose (CP), cellulose acetatebutyrate (CAB), cellulose nitrate (CN)], fluorin based polymers[polychlorofluoroethylene (PCTFE), polytetrafluoroethylene (PTFE),tetrafluoroethylene-hexafluoroethylene copolymer (FEP), poly(vinylidenefluoride) (PVdF)], urethane based polymers (PU), nylons [type 6, type66, type 610, type 11], polyesters (alkyl) [polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polycyclohexane terephthalate(PCT)], novolac type phenolic resins, or the like. In addition,thermosetting resins such as resol type phenolic resin, a urea resin, amelamine resin, a polyurethane resin, an epoxy resin, an unsaturatedpolyester and the like, and natural resins such as protein, gum,shellac, copal, starch and rosin may also be used.

Furthermore, to the binder, a plasticizer for stabilizing theflexibility and strength of the print film and a solvent for adjustingthe viscosity and drying property thereof may be added according to theneeds therefor. The solvent may comprise any one or more of an ester,such as n-propyl acetate, iso-propyl acetate, ethyl acetate, butylacetate; an alcohol, such as ethyl alcohol, industrial methylatedspirits, isopropyl alcohol or normal propyl alcohol; a ketone, such asmethyl ethyl ketone or acetone; an aromatic hydrocarbon, such as xyleneand toluene. A solvent of a low boiling temperature of about 100° C. anda petroleum solvent of a high boiling temperature of 250° C. or higher,may be used according to the type of the printing method. Analkylbenzene or the like, for example may be used as a solvent of a lowboiling temperature. Examples of solvents are ethoxypropanol,methylethylketon, methoxypropylacetate, diacetonalcohol etc.

Further in addition, an auxiliary agent including a variety of reactiveagents for improving drying property, viscosity, and dispersibility, maysuitably be added. The auxiliary agents are to adjust the performance ofthe ink, and for example, a compound that improves the abrasionresistance of the ink surface and a drying agent that accelerates thedrying of the ink, and the like may be employed.

A photopolymerization-curable resin or an electron beam curable resinwherein a solvent is not used may also be employed as a binder resinthat is a principal component of the vehicle. The examples thereofinclude an acrylic resin, and specific examples of acrylic monomerscommercially available are shown below.

A monofunctional acrylate monomer that may be used includes for example,2-ethylhexyl acrylate, 2-ethylhexyl-EO adduct acrylate, ethoxydiethyleneglycol acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,2-hydroxyethyl acrylate-caprolactone addduct, 2-phenoxyethyl acrylate,phenoxydiethylene glycol acrylate, nonyl phenol-EO adduct acrylate,(nonyl phenol-EO adduct)-caprolactone adduct acrylate,2-hydroxy-3-phenoxypropyl acrylate, tetrahydrofurfuryl acrylate,furfuryl alcohol-caprolactone adduct acrylate, acryloyl morpholine,dicyclopentenyl acrylate, dicyclopentanyl acrylate,dicyclopentenyloxyethyl acrylate, isobornyl acrylate,(4,4-dimethyl-1,3-dioxane)-caprolactone adduct acrylate,(3-methyl-5,5-dimethyl-1,3-dioxane)-caprolactone adduct acrylate, andthe like.

A polyfunctional acrylate monomer that may be used includes hexanedioldiacrylate, neopentyl glycol diacrylate, polyethylene glycol diacrylate,tripropylene glycol diacrylate, neopentyl glycol hydroxypivalatediacrylate, (neopentyl glycol hydroxypivalate)-caprolactone adductdiacrylate, (1,6-hexanediol diglycidyl ether)-acrylic acid adduct,(hydroxypivalaldehyde-trimethylolpropane acetal) diacrylate,2,2-bis[4-(acryloyloxydiethoxy)phenyl]propane,2,2-bis[4-(acryloyloxydiethoxy)phenyl]methane, hydrogenated bisphenolA-ethylene oxide adduct diacrylate, tricyclodecanedimethanol diacrylate,trimethylolpropane triacrylate, pentaerithritol triacrylate,(trimethylolpropane-propylene oxide) adduct triacrylate,glycerine-propylene oxide adduct triacrylate, a mixture ofdipentaerithritol hexaacrylate and pentaacrylate, esters ofdipentaerithritol and lower fatty acid and acrylic acid,dipentaerithritol-caprolactone adduct acrylate, tris(acryloyloxyethyl)isocyanurate, 2-acryloyloxyethyl phosphate, and the like.

Inks comprising the above resins are free of solvent and are soconstituted as to polymerize in chain reaction upon irradiation by anelectron beam or electromagnetic waves.

With respect to inks of ultraviolet-irradiation type among these inks, aphotopolymerization initiator, and depending on the needs therefor, asensitizing agent, and auxiliary agents such as a polymerizationinhibitor and a chain transfer agent, and the like may be added thereto.

With respect to photo-polymerization initiators, there are, (1) aninitiator of direct photolysis type including an arylalkyl ketone, anoxime ketone, an acylphosphine oxide, or the like, (2) an initiator ofradical polymerization reaction type including a benzophenonederivative, a thioxanthone derivative, or the like, (3) an initiator ofcationic polymerization reaction type including an aryl diazonium salt,an aryl iodinium salt, an aryl sulfonium salt, and an aryl acetophenonesalt, or the like, and in addition, (4) an initiator of energy transfertype, (5) an initiator of photoredox type, (6) an initiator of electrontransfer type, and the like. With respect to the inks of electronbeam-curable type, a photopolymerization initiator is not necessary anda resin of the same type as in the case of the ultraviolet-irradiationtype inks can be used, and various kinds of auxiliary agent may be addedthereto according to the needs therefor.

The inks comprise a total content of shaped transition metal particlesof from 0.1 to 90% by weight, preferably 0.1-70% by weight based on thetotal weight of the ink.

Preferably, the binder comprises 50% nitrocellulose in conjunction withany above mentioned resin.

The composition may additionally comprise a solvent. The solvent may beester/alcohol blends and preferably normal propyl acetate and ethanol.More preferably, the ester/alcohol blend is in a ratio of between 10:1and 40:1, even more preferably 20:1 to 30:1.

The solvent used in the metallic ink may comprise any one or more of anester, such as n-propyl acetate, iso-propyl acetate, ethyl acetate,butyl acetate; an alcohol, such as ethyl alcohol, industrial methylatedspirits, isopropyl alcohol or normal propyl alcohol; a ketone, such asmethyl ethyl ketone or acetone; an aromatic hydrocarbon, such astoluene, and water.

Preferably, the composition is used in the manufacture of a hologram.Reference is made to WO2005/051675 and WO2008/061930. The methoddescribed therein for forming an optically variable image (an opticallyvariable device) on a substrate comprises the steps of:

-   a) forming an optically variable image (OVI) on a discrete portion    of the substrate; and-   b) depositing a coating composition, especially a metallic ink,    comprising platelet shaped transition metal particles having a    longest dimension of edge length of from 15 nm to 1000 nm,    preferably from 15 nm to 600 nm and particularly from 20 nm to 500    nm, and a thickness of from 2 nm to 100 nm, preferably from 2 to 40    nm and particularly from 4 to 30 nm and a binder on at least a    portion of the OVI.

Preferably, step a) comprises

-   a1) applying a curable compound to at least a portion of the    substrate;-   a2) contacting at least a portion of the curable compound with OVI    forming means; and-   a3) curing the curable compound.

Referring to FIG. 1 of WO08/061930, paper, aluminium, or another opaquesubstrates (1) is printed with an ultra violet curable lacquer (2) onits lower surface. An optically variable device or other lens orengraved structure is cast (3) into the surface of the lacquer (2) witha clear shim (4) having the optically variable device or other lens orengraved structure thereon. The optically variable device or other lensor engraved structure image is imparted into the lacquer and instantlycured (6) via an UV lamp disposed through the shim (4) at normalprocessing speeds through polarizing lens (8), quartz roller (6), andclear polycarbonate roller (5). The optically variable device or otherlens or engraved structure image is a facsimile of the image on theclear shim. Metallic ink (9) is printed (10) over the optically variabledevice or other lens or engraved structure and causes the opticallyvariable device or other lens or engraved structure to become lightreflective. Further colours (11) can be subsequently conventionallyprinted in-line at normal printing process speeds. In an alternativeembodiment, the paper, aluminium, and all manner of other opaquesubstrate (1) is replaced with a filmic substrate. Such material issubstantially transparent and therefore the image is visible from bothsides of the surface.

The (security) product obtainable by using the above method is new andforms a further subject of the present application.

In said embodiment (security) product comprises a substrate, an UV curedlacquer, holographic or other sub-microscopic diffraction gratingsengraved into the UV cured lacquer and a metallic coating on theengraved UV cured lacquer on at least portion of the substrate.

Referring to FIG. 12 of WO08/061930 a film substrate 100, UV curedlacquer 102 and holographic or other sub-microscopic diffraction grating104 with metallic ink 106 printed over with both first 108 and secondsurfaces 110 viewable.

Referring to FIG. 13 of WO08/061930 a paper substrate 120, UV curedlacquer 122 and holographic or other sub-microscopic diffraction grating124 with metallic ink 126 printed over with the image viewable throughthe first surface 128 only.

In a further preferred embodiment of the present invention a coloured,or metallic ink is deposited on a substrate, on which the opticallyvariable image is formed; before forming the optically variable image onat least a portion of the coloured, or metallic ink.

The substrate may comprise any sheet material. The substrate may beopaque, substantially transparent or translucent, wherein the methoddescribed in WO08/061930 is especially suited for substrates, which areopaque to UV light (non-transparent). The substrate may comprise paper,leather, fabric such as silk, cotton, tyvac, filmic material or metal,such as aluminium. The substrate may be in the form of one or moresheets or a web.

The substrate may be mould made, woven, non-woven, cast, calendared,blown, extruded and/or biaxially extruded. The substrate may comprisepaper, fabric, man made fibres and polymeric compounds. The substratemay comprise any one or more selected from the group comprising paper,papers made from wood pulp or cotton or synthetic wood free fibres andboard. The paper/board may be coated, calendared or machine glazed;coated, uncoated, mould made with cotton or denim content, Tyvac, linen,cotton, silk, leather, polythyleneterephthalate, polypropylenepropafilm, polyvinylchloride, rigid PVC, cellulose, tri-acetate, acetatepolystyrene, polyethylene, nylon, acrylic and polytherimide board. Thepolythyleneterephthalate substrate may be Melienex type film orientatedpolypropylene (obtainable from DuPont Films Willimington Del. product IDMelinex HS-2).

The substrates being transparent filmic or non transparent substrateslike opaque plastic, paper including but not limited to banknote,voucher, passport, and any other security or fiduciary documents, selfadhesive stamp and excise seals, card, tobacco, pharmaceutical, computersoftware packaging and certificates of authentication, aluminium, andthe like.

In a preferred embodiment of the present invention the substrate is anon-transparent (opaque) sheet material, such as, for example, paper. Inanother preferred embodiment of the present invention the substrate is atransparent sheet material, such as, for example,polythyleneterephthalate.

The forming of an optically variable image on the substrate may comprisedepositing a curable compound, or composition on at least a portion ofthe substrate. The composition, generally a coating or lacquer may bedeposited by means of gravure, flexographic, ink jet and screen processprinting. The curable lacquer may be cured by actinic radiations,preferably ultraviolet (U.V.) light or electron beam. Preferably, thelacquer is UV cured. UV curing lacquers can be obtained from BASF SE.The lacquers exposed to actinic radiations or electron beam used in thepresent invention are required to reach a solidified stage when theyseparate again from the imaging shim in order to keep the record intheir upper layer of the sub-microscopic, holographic diffractiongrating image or pattern (OVI). Particularly suitable for the lacquerscompositions are chemistries used in the radiation curable industries inindustrial coatings and graphic arts. Particularly suitable arecompositions containing one or several photo-latent catalysts that willinitiate polymerization of the exposed lacquer layer to actinicradiations. Particularly suitable for fast curing and conversion to asolid state are compositions comprising one or several monomers andoligomers sensitive to free-radical polymerization, such as acrylates,methacrylates or monomers or/and oligomers, containing at least oneethylenically unsaturated group. Reference is made to pages 8 to 35 ofWO2008/061930.

The UV lacquer may comprise an epoxy-acrylate from the CRAYNOR® SartomerEurope range (10 to 60%) and one or several acrylates (monofunctionaland multifunctional), monomers which are available from Sartomer Europe(20 to 90%) and one, or several photoinitiators (1 to 15%) such asDarocure® 1173 and a levelling agent such as BYK®361 (0.01 to 1%) fromBYK Chemie.

The curable composition is preferably deposited by means of gravure orflexographic printing.

The curable composition is preferably curable by means of an ultraviolet(U.V.) light or an electron beam. The curable composition can becoloured.

The metallic ink may be applied to the substrate by means ofconventional printing press such as gravure, rotogravure, flexographic,lithographic, offset, letterpress intaglio and/or screen process, orother printing process.

In order that the hologram is clearly visible on both the first andsecond surface of a clear filmic substrate and the first surface of apaper substrate, preferably, the metallic pigment particles are printedin such a way as to align themselves such that they follow and conformto the contours of the diffraction grating.

To accomplish this alignment of metal pigment particles to the contoursof the diffraction grating the ink (coating composition) preferably hasa very low binder, a low pigment content and a medium pigment to binderratio and/or very thin pigment particles.

A filmic substrate is printed conventionally with a number of colouredinks, using, for example, a Cerutti R950 printer (available fromCerrutti UK Long Hanborough Oxon.). The substrate is then printed withan ultra violet curable lacquer. An OVI is cast into the surface of thecurable composition with a shim having the OVI thereon, the holographicimage is imparted into the lacquer and instantly cured via a UV lamp,becoming a facsimile of the OVI disposed on the shim. A metallic ink isprinted over the OVI and causes the OVI to become light reflective, theOVI is visible on the first surface of a paper or other non-filmicsubstrate and both sides of the filmic substrate. In another embodiment,the UV curable composition is replaced with an electronic beam curablecomposition and the UV lamp replaced with an electron beam emittingdevice.

The ink preferably comprises low solids, high viscosity binders.Preferably, the pigment to binder ratio is in the range of 10:1 to 1:10by weight. More preferably, the pigment to binder ratio is by weight inthe range of 6:1 to 1:6, and even more preferably 4:1 to 1:4. Mostpreferably the pigment to binder ratio is from 3:1 to 1:3.

The metal pigment content by weight of the composition may be less than90%.

Preferably the pigment content by weight of the composition is less than70%, more preferably in the range of 0.1% to 90%, even more preferablyin the range 0.1% to 70%.

The binder may comprise any one or more selected from the groupcomprising nitro cellulose, vinyl chloride, vinyl acetate copolymers,vinyl, acrylic, urethane, polythyleneterephthalate, terpene phenol,polyolefin, silicone, cellulose, polyamide, polyester, rosin esterresins. The preferred binder is 50% nitrocellulose (ID nitrocelluloseDHL120/170 and nitrocellulose DLX30/50 supplied by Nobel Industries) 50%polyurethane (ID Neorez U335 supplied by Avecia). The solvents may beester/alcohol blends and preferably normal propyl acetate and ethanol ina ratio of 20:1 to 30:1.

The compositions of the present invention can be applied to hologramsfor use on substrates such as (security products), including banknotes,credit cards, identification documents like passports, identificationcards, drivers licenses, or other verification documents, pharmaceuticalapparel, software, compact discs, tobacco packaging and other productsor packaging prone to counterfeiting or forgery, to protect them fromfraudulent conversion, diversion or imitation.

Preferably, when the substrate carrying the metallised image or patternis subsequently over-laid onto printed pictures and/or text, or thesubstrate is pre-printed with pictures and/or text and the metallisedimage or pattern is deposited thereon those pre-printed features arevisible through the substrate and/or the metallic composition coateddiffraction grating or image.

The coating compositions may be deposited on a diffraction gratingdisposed on a substrate such as a substantially transparent,translucent, or opaque substrate. The substrate may comprise paper,filmic material or metal, such as aluminium.

The substrate may comprise polymeric compounds. The substrate maycomprise papers made from wood pulp or cotton or synthetic wood-freefibres.

The diffraction grating may be formed using any methods known to theskilled man such as those described in U.S. Pat. No. 4,913,858, U.S.Pat. No. 5,164,227, WO2005/051675 and WO2008/061930.

The coating composition may be applied to the substrate by means ofconventional printing press such as gravure, rotogravure, flexographic,lithographic, offset, letterpress intaglio and/or screen process, orother printing process.

Preferably, when the substrate carrying the enhanced diffractive imageor pattern is subsequently over-laid onto printed pictures and/or text,or the substrate is pre-printed with pictures and/or text and theenhanced diffractive image or pattern is deposited thereon, thoseprinted features are visible through the substrate and/or the metallicink coated diffraction grating or image.

By varying the film-weight and density of the deposited metallic ink,the transmission of light through the enhanced image can be adjusted toprovide a desirable range of visual effects.

The composition may further comprise modifying additives, for examplecolorants and/or suitable solvent (s).

Preferably, the resin maintains adhesion of the composition to thesurface of the diffraction grating.

Specific additives can be added to the composition to modify itschemicals and/or physical properties. Polychromatic effects can beachieved by the introduction of (colored) inorganic and/or organicpigments and/or solvent soluble dyestuffs into the ink, to achieve arange of coloured shades. By addition of a dye the transmission colourcan be influenced. By the addition of fluorescent, or phosphorescentmaterials the transmission and/or the reflection colour can beinfluenced.

Suitable colored pigments especially include organic pigments selectedfrom the group consisting of azo, azomethine, methine, anthraquinone,phthalocyanine, perinone, perylene, diketopyrrolopyrrole, thioindigo,dioxazine iminoisoindoline, dioxazine, iminoisoindolinone, quinacridone,flavanthrone, indanthrone, anthrapyrimidine and quinophthalone pigments,or a mixture or solid solution thereof; especially a dioxazine,diketopyrrolopyrrole, quinacridone, phthalocyanine, indanthrone oriminoisoindolinone pigment, or a mixture or solid solution thereof.

Colored organic pigments of particular interest include C.I. Pigment Red202, C.I. Pigment Red 122, C.I. Pigment Red 179, C.I. Pigment Red 170,C.I. Pigment Red 144, C.I. Pigment Red 177, C.I. Pigment Red 254, C.I.Pigment Red 255, C.I. Pigment Red 264, C.I. Pigment Brown 23, C.I.Pigment Yellow 109, C.I. Pigment Yellow 110, C.I. Pigment Yellow 147,C.I. Pigment Orange 61, C.I. Pigment Orange 71, C.I. Pigment Orange 73,C.I. Pigment Orange 48, C.I. Pigment Orange 49, C.I. Pigment Blue 15,C.I. Pigment Blue 60, C.I. Pigment Violet 23, C.I. Pigment Violet 37,C.I. Pigment Violet 19, C.I. Pigment Green 7, C.I. Pigment Green 36, the2,9-dichloro-quinacridone in platelet form described in WO08/055807, ora mixture or solid solution thereof.

Plateletlike organic pigments, such as plateletlike quinacridones,phthalocyanine, fluororubine, dioxazines, red perylenes ordiketopyrrolopyrroles can advantageously be used.

Suitable colored pigments also include conventional inorganic pigments;especially those selected from the group consisting of metal oxides,antimony yellow, lead chromate, lead chromate sulfate, lead molybdate,ultramarine blue, cobalt blue, manganese blue, chrome oxide green,hydrated chrome oxide green, cobalt green and metal sulfides, such ascerium or cadmium sulfide, cadmium sulfoselenides, zinc ferrite, bismuthvanadate, Prussian blue, Fe₃O₄, carbon black and mixed metal oxides.

Examples of dyes, which can be used to color the curable composition,are selected from the group consisting of azo, azomethine, methine,anthraquinone, phthalocyanine, dioxazine, flavanthrone, indanthrone,anthrapyrimidine and metal complex dyes. Monoazo dyes, cobalt complexdyes, chrome complex dyes, anthraquinone dyes and copper phthalocyaninedyes are preferred.

It is possible to generate striking colour effects by combining shaped,especially platelet-like and spherical silver particles having adiameter in the range of from 3 to 40 nm. Reference is made to Example 7of the present application.

Hence, in a preferred embodiment of the present invention the coatingcomposition comprises besides platelet-like transition metal (silver)particles spherical transition metal (silver) particles having adiameter in the range of from 3 to 40 nm, wherein up to 40% of thetransition metal (silver) particles can be spherical.

The binder resins may be initially dissolved in the appropriatesolvent(s) to form liquid varnishes. These varnishes can then be blendedtogether with the metallic pigment and/or other components by means of ahigh-speed blender to produce the composition.

In accordance with a further aspect of the present invention, there isprovided a metallic ink comprising shaped transition metal particles(pigment) and a binder. The ratio of pigment to binder is sufficientlyhigh as to permit the alignment of the metal particles to the contoursof a diffraction grating.

The shaped transition metal particles may be prepared by any means knownto the skilled man. For example, US2008/0295646 describes a thermalmethod of preparing metal, in particular silver nanoprisms having aunimodal size distribution and a predetermined thickness in the form ofa colloidal suspension. A photochemical method of preparing silvernanoprisms of controlled edge length through wavelength modulation isdescribed in WO2004/089813.

WO2006/099312 describes the synthesis of gold nanoprisms. Silica-coatedsilver prisms which can be dispersed in a variety of organic solventsare described in C. Xue et al., Adv. Mater. 19, 2007, 4071.

WO2009056401 describes nano-shaped transition metal particles, inparticular nanoplatelets, characterized by a surface plasmon resonancein the near infrared (NIR) range and their preparation. These particlesare used as IR absorbers in heat shielding architectural, automotiveglazing or agricultural films, laser welding, laser printing, securityprinting and near infrared curing of coatings.

The shaped transition metal particles may be prepared by any means knownto the person skilled in the art. Reference is, for example, made toWO2010/108837. The method of manufacturing shaped transition metalparticles, selected from the group consisting of Cu, Ag, Au, Zn, Cd, Ti,Cr, Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt, comprises the steps offirst

a) generating isotropic spherical metal nanoparticles by mixing asolution of reducing agent with an aqueous mixture comprising atransition metal salt in a concentration of higher than 2 mmol per literand a polymeric dispersant and/or capping agent, and

b) inducing controlled agglomeration of said isotropic nanoparticles bytreatment with a peroxide, leading to formation of platelet-shapedmetallic nano- or microparticles.

The transition metal is preferably Ag, Cu, Pd or Au, more preferably Ag.The shaped particles may also be made from two of the above-mentionedtransition metals to form core-shell or alloy type particles, forexample as described in WO07103536.

Suitable examples of silver salts of mono-, di-, tri- or polycarboxylicacids include silver salts of acetic acid, propionic acid, 4-cyclohexylbutyric acid, oxalic acid, malonic acid, succinic acid, malic acid,maleic acid, fumaric acid, glutaric acid, adipic acid, citric acid andpolyacrylic acid.

Suitable examples of silver salts of sulfonic or polysulfonic acidsinclude silver salts of methane sulfonic acid, trifluormethane sulfonicacid, vinyl sulfonic acid, benzene sulfonic acid, toluene sulfonic acid,styrene sulfonic acid and sulfonated polystyrene.

Suitable examples of silver salts of P-containing acids include silversalts of phosphoric acid, metaphosphoric acid, phosphorous acid,pyrophosphoric acid, hypophosphoric acid and organo-substitutedderivatives thereof, phenolphosphate resins, polyacrylic phosphates andphosphonates.

Preferred silver(I) salts are AgNO₃, Ag₂O, AgClO₄, Ag₂SO₄, CH₃CO₂Ag,mono-, di- or trisilver citrate, CH₃SO₃Ag, CF₃SO₃Ag, wherein AgNO₃,CH₃CO₂Ag and Ag₂O are more preferred.

Examples of suitable gold salts are KAu(CN)₂; AuI; AuBr; AuCl; R¹CO₂Au,wherein R¹ has the same meaning, as described for R¹CO₂Ag; HAuCl₄;AuBr₃; AuBr₄K; AuBr₄Na; AuCl₃; AuCl₄K; AuCl₄Li; AuCl₄Na and mixturesthereof, wherein HAuCl₄ is preferred.

Examples of suitable copper salts are Cu(NO₃)₂; KCu(CN)₂;copper(II)acetylacetonate; Cu(R¹CO₂)₂, wherein R¹ has the same meaning,as described for R¹CO₂Ag; Cu(ClO₄)₂; CuBr, CuBr₂, CuCl, CuCl₂, CuI,CuSO₄ and mixtures thereof.

The dispersant may be any polymer which prevents agglomeration oraggregation of the spherical and shaped particles. The dispersant may bea non-ionic, anionic or cationic polymer having a weight averagemolecular weight of from 500 to 2,000,000 g/mol, preferably from 1500 to1,000,000, which forms a solution or emulsion in the aqueous mixture.Typically, the polymers may contain polar groups. Suitable polymericdispersants often possess a two-component structure comprising apolymeric chain and an anchoring group. The particular combination ofthese leads to their effectiveness.

Suitable commercially available polymeric dispersants are, for example,EFKA® 4046, 4047, 4060, 4300, 4330, 4580, 4585, 8512, Disperbyk® 161,162, 163, 164, 165, 166, 168, 169, 170, 2000, 2001, 2050, 2090, 2091,2095, 2096, 2105, 2150, Ajinomoto Fine Techno's PB® 711, 821, 822, 823,824, 827, Lubrizol's Solsperse® 24000, 31845, 32500, 32550, 32600,33500, 34750, 36000, 36600, 37500, 39000, 41090, 44000, 53095,ALBRITECT® CP30 (a copolymer of acrylic acid and acrylphosphonate) andcombinations thereof.

Preference is given to polymers derived from hydroxyalkyl(meth)acrylatesand/or polyglycol (meth)acrylates, such as hydroxyethyl andhydroxypropyl (meth)acrylate, polyethylene glycol (meth)acrylates,(meth)acrylates having amine functionality, for example,N-[3-(dimethylamino)propyl](meth)acrylamide or2-(N,N-dimethylamino)ethyl(meth)acrylate.

In particular, non-ionic copolymer dispersants having aminefunctionality are preferred. Such dispersants are commerciallyavailable, for example as EFKA® 4300, EFKA® 4580 or EFKA 4585.

The polymeric dispersants may be used alone or in admixture of two ormore.

Suitable reducing agents may be selected from the group consisting ofboranes and complexes thereof, metal boranates, hydrides, aluminates,aldehydes, carboxylic acids, hydrazines, hydrosulfites, stannanes,stannates, silanes, phosphines, phosphites and siloxanes.

Preference is given to sodium borohydride, borane complexes withsulfides and amines, hydrazine and ascorbic acid.

Examples of suitable peroxides are selected from the group consisting ofH₂O₂, C₁-C₈alkyl peroxyacids, e.g. peracetic acid, acetyl cyclohexanesulfonyl peroxide, diisopropyl peroxydicarbonate, tert-amylperneodecanoate, tert-butyl perneodecanoate, tert-butyl perpivalate,tert-amylperpivalate, bis(2,4-dichlorobenzoyl)peroxide, diisononanoylperoxide, didecanoyl peroxide, dioctanoyl peroxide, dilauroyl peroxide,bis(2-methylbenzoyl)peroxide, disuccinic acid peroxide, diacetylperoxide, dibenzoyl peroxide, tert-butyl per-2-ethylhexanoate,bis(4-chlorobenzoyl)-peroxide, tert-butyl perisobutyrate, tert-butylpermaleinate, 1,1-bis(tert-butylperoxy)3,5,5-trimethylcyclohexane,1,1-bis(tert-butylperoxy)cyclohexane, tert-butyl peroxyisopropylcarbonate, tert-butyl perisononaoate, 2,5-dimethylhexane2,5-dibenzoate, tert-butyl peracetate, tert-amyl perbenzoate, tert-butylperbenzoate, 2,2-bis(tert-butylperoxy)butane, 2,2 bis(tert-butylperoxy)propane, dicumyl peroxide, 2,5-dimethylhexane-2,5-di-tert-butylperoxide,3-tert-butylperoxy 3-phenylphthalide, di-tert-amyl peroxide,α,α′-bis(tert-butylperoxyisopropyl)benzene, 3,5-bis(t-butylperoxy)3,5-dimethyl 1,2-dioxolane, di-tert-butylperoxide,2,5-dimethylhexyne-2,5-di-tert-butylperoxide and3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxacyclononane, wherein H₂O₂ ispreferred.

Optionally, the mixture of step a) of the present invention contains acapping agent. If present, the capping agent is typically used in aconcentration up to 1 M, preferably from 0.001 to 100 mM.

Examples of suitable capping agents include mono- and polycarboxylicacids (e.g. citric acid, ethylenediamine tetraacetic acid, propionicacid) and salts thereof (e.g. sodium citrate, sodium salts ofethylenediamine tetraacetic acid, sodium propionate), sulphur-containingcompounds, such as thiols (e.g. mercaptoethanol, dithiothreitol,mercaptopolyethyleneglycol), linear and cyclic disulfides (e.g.cystamine, mercaptoethanol disulfide, cyclo-dithiothreitol), xanthates(e.g. sodium ethylxanthogenate), dithiocarbamates (e.g. diethylammoniumsalt of diethyldithiocarbamic acid) and the like, amines (e.g. ammonia,aminoethanol, methylamine) and aminocarboxylic acids (e.g. aminoaceticacid, 2-aminopropionic acid, 3-aminopropionic acid).

The process of platelet-shaped particles manufacture is preferablycarried out by dissolving or dispersing the transition metal salt inwater in concentration of >0.2% by weight in the presence of adispersant and optionally at least one of abovementioned water-solublecapping agents and dissolving or dispersing the reducing agent andoptionally at least one of abovementioned water-soluble capping agentsin water. The resulting solutions or dispersions are held at atemperature of less than 20° C., preferably they are cooled down to 5°C. or less, e.g. about 0 to 5° C. These two solutions are mixed eitherby addition of one solution to another, or, preferably, they are mixedby pumping both solutions into a mixing chamber, said chamber havingadditional outlet to collect the resulting dispersion of sphericalnanoparticles. The simplest example of such a mixing chamber is athree-way connector of any shape. Upon completion of the metal saltreduction, the >20% by weight solution of hydrogen peroxide in water isslowly added to the dispersion of spherical metal nanoparticles withstirring until the desired spectral profile of reaction mixture isachieved. The step of peroxide addition may be performed at atemperature of from about 0° C. to about 100° C. Preferably, theperoxide is added at a temperature of from 20 to 70° C., more preferablyfrom 30 to 60° C. The as prepared platelet-shaped metal particles areisolated by any means, known to a person skilled in the art, e.g.reversible agglomeration using appropriate surface-modifying agentsand/or by centrifugation and/or by extraction with organic solvent.

The shaped particles may show NIR absorption, which characteristic mightrepresent an additional security feature.

The coating composition, especially metallic ink of the presentinvention can also be used in the production of a hot stamping foil.

Accordingly, the present invention is also directed to a method ofproducing a hot stamping foil comprising the steps of:

(a) coating a carrier with a release coating,(b) applying a coating of a hard lacquer onto the release coating,(c) applying an ultraviolet primer coating onto the coating of the hardlacquer,(d) contacting at least a portion of the ultraviolet primer coating withoptically variable image (optically variable device) forming means;(e) depositing a coating composition comprising platelet shapedtransition metal particles having a longest dimension of edge length offrom 15 nm to 1000 nm, preferably from 15 nm to 600 nm and particularlyfrom 20 nm to 500 nm, and a thickness of from 2 nm to 100 nm, preferablyfrom 2 to 40 nm and particularly from 4 to 30 nm and a binder,especially a printing ink to the UV primer holding the transferredoptically variable device either as a whole or in partial areas,(f) optionally printing subsequently process colours, and(g) applying a heat activated adhesive onto the layer obtained in stepe), or f).

Examples of the release compound are silica, microcrystalline wax, ricewax, oricuri wax, stearic acid esters, polyglycols, and metallic saltsof fatty acids.

Examples of the coating of the hard lacquer are polymethylmethacrylate,styrene acrylonitrile, polyethyleneterephthalate, nitrocellulose, ormixtures thereof. The coating of the hard lacquer affixed to saidrelease coating has in general a thickness in the range 0.25 microns to9 microns and has a glass transition temperature of at least 70° C.

Examples of the adhesive compound are vinyl alcohol, polyacrylates,polyalkacrylates, vinyl resins, polyvinyl acetate, cellulose resins,polyacrylamides, and ethylene/vinyl acetate copolymers.

Under the present invention a carrier film substrate having thethickness on the order of 12 microns to 75 microns and formed of asuitable plastic material such as a polyester, oriented polypropylene orother suitable material is coated with a release coating such as amicrocrystalline wax or a partially saponified montan wax or other waxbased coatings having a thickness in the range of 0.025 microns to 5microns and then has a coating of hard lacquer applied over the releasecoating in a thickness in the range of 0.25 microns to 10 microns. Thehard lacquer coating may be applied by a gravure roller following whichis dried an ultraviolet primer coating having the thickness in a rangeof 0.3 microns to 9 microns is then applied by means of a gravureroller. An optically variable device is contacted by the embossing shimhaving an optically variable device thereon and is transferred into thesurface of the UV lacquer and cured by UV light. A layer of the vacuummetallic ink is applied to the UV primer holding the transferredoptically variable device either as a whole or in partial areas.Subsequent process colours can be gravure printed.

The hot stamping foil obtained in the above process can be used to labelan article of manufacture. The method of labeling the article ofmanufacture comprises the steps of: contacting the heat activatedadhesive layer of the hot stamping foil obtained according to the aboveprocess with said article; hot stamping said hot stamping foil to causesaid heat activated adhesive layer to adhere to said article; andremoving the carrier of said hot stamping foil from said hard lacquerlayer.

The optically variable device (OVD) is, for example, an diffractiveoptical variable image (DOVI). The term “diffractive optical variableimage” as used herein may refer to any type of holograms including, forexample, but not limited to a multiple plane hologram (e.g.,2-dimensional hologram, 3-dimensional hologram, etc.), a stereogram, anda grating image (e.g., dot-matrix, pixelgram, exelgram, kinegram, etc.).

Examples of an optically variable image or device are holograms ordiffraction gratings, moire grating, etc. These optical microstructuredimages are composed of a series of structured surfaces. These surfacesmay have straight or curved profiles, with constant or random spacing,and may even vary from microns to millimetres in dimension. Patterns maybe circular, linear, or have no uniform pattern. For example a Fresnellens has a microstructured surface on one side and a pano surface on theother. The microstructured surface consists of a series of grooves withchanging slope angles as the distance from the optical axis increases.The draft facets located between the slope facets usually do not affectthe optical performance of the Fresnel lens.

Another aspect of the present invention is directed to a decorative, orsecurity element, especially a security element which displays a moreevident visual colour change and which can easily be checked by the “manon the street”.

Accordingly, the present invention relates to a security element,especially for a document of value, right, identity, for a securitylabel or a branded good, comprising a substrate, which may containindicia or other visible features in or on its surface, and on at leastpart of the said substrate surface, a coating comprising platelet shapedtransition metal particles having a longest dimension of edge length offrom 15 nm to 1000 nm, preferably from 15 nm to 600 nm and particularlyfrom 20 nm to 500 nm, and a thickness of from 2 nm to 100 nm, preferablyfrom 2 to 40 nm and particularly from 4 to 30 nm.

Preferably, the coating shows an angle dependent colour change. Thecolour change is a “simple message of authenticity”, which can bechecked by the unaided human eye.

At least part of a substrate is coated with a layer, comprising theplatelet shaped transition metal particles. The coating comprising theplatelet shaped transition metal particles can have a form. The form maybe, for example, a symbol, a stripe, a geometric form, a fancy emblem, awriting, an alphanumeric character, a depiction of an object, or a partthereof.

The security element of the present invention can be used for theprevention of counterfeit or reproduction, on a document of value,right, identity, a security label or a branded good.

Using coating compositions resulting in different colour flop thesecurity element according to invention can be so constructed that twoadjacent portions of a coating show different colours depending on theviewing angles.

In a preferred embodiment the first surface portion shows a first colourat a first viewing angle, while the second surface portion shows asecond colour at the first viewing angle. At a second viewing angle thefirst surface portion shows the second colour and the second surfaceportion shows the first colour. Preferably the colours used exhibit ahigh colour contrast.

A method of producing the security element, comprises the steps of

a) providing a substrate having a surface, which surface may containindicia or other visible features;

b) applying, on top of at least part of the said substrate surface acoating, comprising the platelet shaped transition metal particles and acurable transparent binder, and (c) curing said coating layer.

As described above, the surface of the substrate may have an OVI on itssurface, or a layer on the substrate may have an OVI on its surface. Thecoating, comprising the platelet shaped transition metal particles and acurable transparent binder, is applied on top of at least part of thesaid OVI.

An object of value, right, identity, security label, branded good, canbe tested for authenticity by checking whether a colour change independence from the viewing angle is present.

Various aspects and features of the present invention will be furtherdiscussed in terms of the examples. The following examples are intendedto illustrate various aspects and features of the present invention, butnot to limited the scope of the present invention.

EXAMPLES Example 1

20 g of the copolymer prepared according to Example 2 from WO2004/045755 A2 (40 w/w dispersion in water), 20 g of ethyleneglycol and6 g of MPEG-5000-thiol are dissolved in 1950 ml of de-ionized water in athermostated 10 l reactor, equipped with an efficient stirrer. Aftercooling to −1° C., 10.2 g (60 mmol) of Ag—NO₃ are added and the obtainedsolution is gently stirred for 15 min. 4.54 g (120 mmol) of NaBH₄ aredissolved in 1 l of de-ionized water in a separate vessel and cooled to0° C. This solution is rapidly added in one portion to the abovesolution of AgNO₃ with vigorous stirring (500 rpm). The reaction mixtureis vigorously stirred (500 rpm) for 5 min at 0° C. and then warmed up to20° C. with gentle stirring over 1 h. 150 ml of H₂O₂ (50% w/w solutionin water) is added at a rate of 3 ml/min to the mixture with vigorousstirring (350 rpm) to obtain a dark-blue dispersion of silver platelets.

Water is evaporated to the volume of 200 ml, and the residual dispersionis centrifuged at 8000 G for 30 min. The supernatant is decanted; theprecipitate is rinsed with de-ionized water (2×40 ml) and re-dispersedin 200 ml of 1,4-dioxan under ultra-sonication. The dispersion iscentrifuged at 8000 G, the supernatant is discarded and the precipitateis re-dispersed in EtOAc to obtain a dispersion of 6.3 g of silverplatelets.

Example 2

Two solutions are prepared:

Solution A: 7 g of the copolymer prepared according to Example 2 from WO2004/045755 A2 (40% w/w dispersion in water) and 1.2 g ofMPEG-5000-thiol are dissolved in 20 ml of de-ionized water and cooled to0° C. Then, a solution of 2.04 g (12 mmol) of AgNO₃ in 23 ml ofde-ionized water is added with stirring, and the resulting mixture iscooled to 0° C.

Solution B: 0.908 g (24 mmol) of NaBH₄ and 0.07 ml (ca. 1.15 mmol) of28% w/w ammonia solution in water are dissolved in 49 ml of de-ionizedwater and cooled to 0° C.

Synthesis: Solutions A and B are pumped with equal flow rates (30ml/min) into a three-way connector (inner diameter of the inlets 1 mm).The resulting dispersion of spherical Ag particles is continuouslydrained from the third outlet (inner diameter 3 mm) of connector into a1 l round-bottom flask, pre-cooled to 0° C., containing 0.05 ml of TEGOFoamex 1488 defoamer and stirred for 10 min at this temperature. Themixture is then heated to 45° C. and treated with 20 ml of H₂O₂ (50% w/wsolution in water) at a rate of 0.6 ml/min with vigorous stirring toobtain a dark-blue dispersion of silver platelets. The dispersion iscentrifuged at 8000 G for 30 min, followed by decanting the supernatantand rinsing the precipitate with de-ionized water (2×40 mL). Theprecipitate is re-dispersed in 200 ml of 1,4-dioxan underultra-sonication. The dispersion is centrifuged at 8000 G, thesupernatant is discarded and the precipitate is re-dispersed in EtOAc toobtain a dispersion of 1.23 g of silver platelets.

The FIGURE view is a Transmission Electron Micrograph (TEM) of theirregularly-shaped silver platelets obtained in Example 2.

Example 3

Two solutions are prepared:

Solution A: 8 g of the copolymer prepared according to Example 2 from WO2004/045755 A2 (40% w/w dispersion in water) and 1.2 g ofMPEG-5000-thiol are dissolved in 20 ml of de-ionized water and cooled to0° C. Then, a solution of 2.04 g (12 mmol) of AgNO₃ in 23 ml ofde-ionized water is added with stirring, and the resulting mixture iscooled to 0° C.

Solution B: 0.908 g (24 mmol) of NaBH₄ and 0.07 ml (ca. 1.15 mmol) of28% w/w ammonia solution in water are dissolved in 49 ml of de-ionizedwater and cooled to 0° C.

Synthesis: Solutions A and B are pumped with equal flow rates (30ml/min) into a three-way connector (inner diameter of the inlets 1 mm).The resulting dispersion of spherical Ag particles is continuouslydrained from the third outlet (inner diameter 3 mm) of connector into a1 l round-bottom flask, pre-cooled to 0° C., containing 0.05 ml of TEGOFoamex 1488 defoamer and stirred for 10 min at this temperature. Themixture is then heated to 45° C. and treated with 20 ml of H₂O₂ (50% w/wsolution in water) at a rate of 0.6 ml/min with vigorous stirring toobtain a dark-blue dispersion of silver platelets. The dispersion iscentrifuged at 8000 G for 30 min, followed by decanting the supernatantand rinsing the precipitate with de-ionized water (2×40 mL). Theprecipitate is re-dispersed in 200 ml of 1,4-dioxan underultra-sonication. The dispersion is centrifuged at 8000 G, thesupernatant is discarded and the precipitate is re-dispersed in EtOAc toobtain a dispersion of 1.23 g of silver platelets.

Example 4

Two solutions are prepared:

Solution A: 10 g of the copolymer prepared according to Example 2 fromWO2004/045755 (40% w/w dispersion in water) and 1.0 g of MPEG-5000-thiolare dissolved in 16 ml of de-ionized water and cooled to 0° C. Then, asolution of 1.70 g (10 mmol) of AgNO₃ in 23 ml of de-ionized water isadded with stirring, and the resulting mixture is cooled to 0° C.

Solution B: 0.756 g (20 mmol) of NaBH₄ and 0.067 ml (1.0 mmol) ofethylenediamine are dissolved in 50 ml of de-ionized water and cooled to0° C.

Synthesis: Solutions A and B are pumped with equal flow rates (30ml/min) into a three-way connector (inner diameter of the inlets 1 mm).The resulting dispersion of spherical silver particles is continuouslydrained from the third outlet (inner diameter 3 mm) of connector into a1 l round-bottom flask, pre-cooled to 0° C., containing 0.05 ml of TEGOFoamex 1488 defoamer and stirred for 10 min at this temperature. Themixture is then heated to 40° C. and treated with 20 ml of H₂O₂ (50% w/wsolution in water) at a rate of 0.5 ml/min with vigorous stirring toobtain a violet dispersion of silver platelets. The dispersion iscentrifuged at 10000 G for 30 min, followed by decanting the supernatantand rinsing the precipitate with de-ionized water (2×40 mL). Theprecipitate is re-dispersed in 200 ml of 1,4-dioxan underultra-sonication. The dispersion is centrifuged at 10000 G, thesupernatant is discarded and the precipitate is re-dispersed in ethylacetate (EtOAc) to obtain a dispersion of 1.02 g of silver platelets.

Example 5

Two solutions are prepared:

Solution A: 10 g of the copolymer prepared according to Example 2 fromWO 2004/045755 (40% w/w dispersion in water) and 1.0 g ofMPEG-5000-thiol are dissolved in 16 ml of de-ionized water and cooled to0° C. Then, a solution of 1.70 g (10 mmol) of AgNO₃ in 23 ml ofde-ionized water is added with stirring, and the resulting mixture iscooled to 0° C.

Solution B: 0.756 g (20 mmol) of NaBH₄ and 43 mg of polyethylene imine(Typical M_(n) 600, Typical M_(w) 800) are dissolved in 50 ml ofde-ionized water and cooled to 0° C.

Synthesis: Solutions A and B are pumped with equal flow rates (30ml/min) into a three-way connector (inner diameter of the inlets 1 mm).The resulting dispersion of spherical Ag particles is continuouslydrained from the third outlet (inner diameter 3 mm) of connector into a1 l round-bottom flask, pre-cooled to 0° C., containing 0.06 ml of TEGOFoamex 1488 defoamer and stirred for 10 min at this temperature. Themixture is then heated to 40° C. and treated with 8.5 ml of H₂O₂ (50%w/w solution in water) at a rate of 0.5 ml/min with vigorous stirring toobtain a magenta-colored dispersion of silver platelets. The dispersionis centrifuged at 10000 G for 30 min, followed by decanting thesupernatant and rinsing the precipitate with de-ionized water (2×40 mL).The precipitate is re-dispersed in 200 ml of 1,4-dioxan underultra-sonication. The dispersion is centrifuged at 10000 G, thesupernatant is discarded and the precipitate is re-dispersed in EtOAc toobtain a dispersion of 1.02 g of silver platelets.

Example 6

Two solutions are prepared:

Solution A: 7 g of the copolymer prepared according to Example 2 from WO2004/045755 A2 (40% w/w dispersion in water) and 1.2 g ofMPEG-5000-thiol are dissolved in 19 ml of de-ionized water and cooled to0° C. Then, a solution of 2.04 g (12 mmol) of AgNO₃ in 23 ml ofde-ionized water is added with stirring, and the resulting mixture iscooled to 0° C.

Solution B: 0.908 g (24 mmol) of NaBH₄ and 0.07 ml (ca. 1.15 mmol) of28% w/w ammonia solution in water are dissolved in 49 ml of de-ionizedwater and cooled to 0° C.

Synthesis: Solutions A and B are pumped with equal flow rates (30ml/min) into a three-way connector (inner diameter of the inlets 1 mm).The resulting dispersion of spherical Ag particles is continuouslydrained from the third outlet (inner diameter 3 mm) of connector into a1 l round-bottom flask, pre-cooled to 0° C., containing 0.06 ml of TEGOFoamex 1488 defoamer and stirred for 10 min at this temperature. Themixture is then ultra-filtered 3 times (300000 polyethersulfonemembrane, 5 bar), the residue is diluted with 1-methoxy-2-propanol(Dowanol) and the mixture is evaporated to the final weight of 6.5 g toprovide 20% w/w dispersion of spherical silver nanoparticles in Dowanol.

Example 7

Varnish preparation: 7.15 g of nitrocellulose (DHM 10-25 IPA (NobelEnterprises, UK)) are slowly added to 92.85 g of ethyl acetate (99-100%rein, Brenntag) in a 250 mL glass bottle and gently stirred untilcomplete dissolution at room temperature. Solid content measurements arethen performed and quantity of ethyl acetate is adjusted to achieve avalue of 10% solid content in the varnish preparation. General procedurefor metallic ink preparation: 3 g silver pigment dispersion (0.6 g ofthe silver particles obtained in Examples 1-6 in 2.4 g ethylacetate) areadded to the above varnish in such a proportion as to adjust the pigmentto binder ratio to the values indicated in Table 1. The obtaineddispersion is stirred with a Dispermat at 800 rpm for 10 minutesaffording a metallic ink which is printed by a handcoater (HC2, 12micron wet film thickness) on contrast paper and a transparent PES film.

TABLE 1 Visual effects obtained with coatings, containing silverparticles obtained according to Examples 1 to 6 of the presentapplication. Silver particles Pigment / Silver obtained BinderReflection Transmission Dispersion in Ex. Ratio Color Color 1 1¹⁾ 3:1.1Gold/Copper Blue 2 2¹⁾ 3:1.1 Gold/Copper Blue 3 3¹⁾ 3:1.1 Gold/CopperBlue 4 4¹⁾ 1:1.4 Gold/Bronze Violet 5 5¹⁾ 1:1.4 Greenish Gold Magenta 66²⁾ 1:3.4 Yellowish Yellowish Brown Brown 7 ³⁾ 2:1   Violet Green 8 1¹⁾1:0.1 Silver Opaque ¹⁾ca. 20% w/w Ag in ethyl acetate. ²⁾ca. 20% w/w Agin Dowanol. ³⁾Mixture of the silver particles obtained in Example 1 and6 in a ratio of 4:1; ca. 20% w/w Ag in ethyl acetate.

The obtained products show depending on the viewing angle a colour flopfrom gold to blue or green to violet or gold to magenta, gold to violet(depending among others on the type of silver particles) above Black andWhite Leneta-Cards.

Example 8

Preparation of UV-curable inks: silver particles obtained in Examples 1to 6 are mixed with BASF SE UV varnish(1-methyl-1,2-ethanediyl)bis[oxy(methyl-2,1-ethanediyl)] diacrylate andpoly(oxy-1,2-ethanediyl), α-hydroxy-[(1-oxo-2-propen-1-yl)oxy]-, etherwith 2-ethyl-2-(hydroxymethyl)-1,3- and propanediol (3:1), Phosphite N,oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone],oxybis(methyl-2,1-ethanediyl)diacrylate, and tertiary amine derivate) insuch a proportion as to adjust the pigment to binder ratio to the valuesindicated in Table 1. Solvent is evaporated at 85° C. in an oven, andthe resulting ink is stirred with a Dispermat at 800 rpm for 10 minutesto afford metallic UV-curable inks, which are printed by a handcoater(HC2, 12 micron film thickness) on transparent PES film and cured at 40m/min, 160 Watt/cm².

Example 9

The silver pigment dispersions (3.4 g of dispersion) as shown in Table 1are mixed with nitrocellulose, n-propylacetate and Dowanol PM (0.25 gnitrocellulose, 2.75 g n-propylacetate, 2.0 g Dowanol PM) to make acoating composition with a pigment to binder ratio shown in Table 1. Thecoatings are printed on an RK proofer press over an aluminium foil, andwhite coated paper provided with a holographic image by applying a cleanUV curable varnish(1-methyl-1,2-ethanediyl)bis[oxy(methyl-2,1-ethanediyl)] diacrylate andpoly(oxy-1,2-ethanediyl), α-hydroxy-[(1-oxo-2-propen-1-yl)oxy]-, etherwith 2-ethyl-2-(hydroxymethyl)-1,3- and propanediol (3:1), Phosphite N,oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone],oxybis(methyl-2,1-ethanediyl)diacrylate, and tertiary amine derivate)onto the corona treated substrate and embossing by using a shim andexposing to UV light.

The obtained products are characterized by an extremely bright OVD imageand extremely strong rainbow effect, high purity and contrast and showdepending on the viewing angle a colour flop from gold to blue or greento violet or gold to magenta, gold to violet (depending among others onthe type of silver particles).

Example 10

Varnish preparation: 10 g of nitrocellulose (DHM 3/5 IPA (NobelEnterprises, UK)) are slowly added to 90.00 g of ethyl acetate (99-100%rein, Brenntag) in a 250 mL glass bottle and gently stirred untilcomplete dissolution at room temperature. Solid content measurermentsare then performed and quantity of ethyl acetate is adjusted to achievea value of 7% solid content in the varnish preparation. Generalprocedure for metallic ink preparation: 28.6 g silver pigment dispersion(16.3 g of the silver particles obtained in ethylacetate) are added tothe above varnish in such a proportion as to adjust 1:1 the pigment tobinder ratio. The obtained dispersion is stirred with a Dispermat at 800rpm for 10 minutes affording a metallic ink which is printed by gravurewith a 100% to 5% step wedge cylinder, 70 l/cm screen on transparent PESand BOPP film. The ink weight printed on film corresponds to 1.26 g/m².

TABLE 1 All colour measurements in remission/transmission are effectedusing a spectrophometer Konica Minolta CM-512M3 (3 angles measurementsat 25°, 45° and 75°) circular illumination over Black&White Leneta cardson the printed surface. Geometry L* C* h° Step wedge 100% PES on black25° 15.3 6.2 68.9 45° 10.8 0.9 105.5 75° 8.5 1.9 263.6 PES on white 25°14.6 10.5 310.9 45° 11.0 13.1 295.8 75° 7.3 11.6 291.6 Step wedge 75%PES on black 25° 13.5 2.9 78.5 45° 10.4 1.6 190.9 75° 6.2 3.6 263.6 PESon white 25° 17.8 27.5 293.7 45° 14.8 29.2 292.0 75° 9.8 25.4 291.9

The obtained prints show depending on the viewing angle a colour flopfrom gold and blue above Black and White Leneta-Cards.

Geometry L* C* h° Step wedge 100% BOPP on black 25° 15.3 6.2 68.9 45°10.8 0.9 105.5 75° 8.5 1.9 263.6 BOPP on white 25° 15.5 17.5 302.7 45°11.8 19.7 295.0 75° 7.2 15.8 292.0 Step wedge 75% BOPP on black 25° 13.52.9 78.5 45° 10.4 1.6 190.9 75° 6.2 3.6 263.6 BOPP on white 25° 17.827.5 293.7 45° 14.8 29.2 292.0 75° 9.8 25.4 291.9

1. A coating composition, comprising: shaped transition metal particleshaving a longest dimension of edge length of from 15 nm to 600 nm and athickness of from 2 to 40 nm; and a binder, wherein a ratio of theshaped transition metal particles to binder is such that a coatingformed from the coating composition shows a variation of color as afunction of the viewing angle, and wherein the transition metal is Ag,Cu, Pd or Au.
 2. The coating composition according to claim 1, whereinthe shaped transition metal particles have a longest dimension of edgelength of from 20 nm to 500 nm and a thickness of from 4 to 30 nm. 3.The coating composition according to claim 1, wherein the transitionmetal is Ag.
 4. The coating composition according to claim 1, whereinthe ratio of binder to shaped transition metal particles is below 10:1.5. The coating composition according to claim 4, wherein the ratio ofbinder to shaped transition metal particles is 3:1 to 1:3.
 6. Thecoating composition according to claim 1, wherein the coating showsdifferent colors in transmission and reflection.
 7. The coatingcomposition according to claim 6, wherein the coating shows a blue colorin transmission and a golden color in reflection.
 8. The coatingcomposition according to claim 7, wherein the transition metal is Ag andan aspect ratio (longest dimension of edge length/thickness) of theshaped transition metal particles is about 6-7.
 9. The coatingcomposition according to claim 1, which is a printing ink.
 10. Thecoating composition according to claim 9, wherein the binder is selectedfrom the group comprising nitro cellulose, vinyl chloride, vinyl acetatecopolymers, vinyl, acrylic, urethane, polythyleneterephthalate, terpenephenol, polyolefin, silicone, cellulose, polyamide, polyester and rosinester resins.
 11. The coating composition according to claim 10, furthercomprising a solvent.
 12. The coating composition according to claim 11,wherein the solvent is a blend of propyl acetate and ethanol in a ratioof 20:1 to
 30. 13. A hologram comprising a coating, wherein the coatingcomprises the coating composition according to claim
 1. 14. A method forforming a coating showing an angle dependent colour change on asubstrate, the method comprising: (a) depositing the coating compositionaccording to claim 1 on the substrate.
 15. The method according to claim11, wherein the depositing is carried out by a gravure, a flexographic,an ink jet, or a screen process printing.
 16. A method of producing asecurity element, the method comprising: a) applying the coatingcomposition according to claim 1 on top of at least part of a substratesurface, to form a coating layer; and b) curing said coating layer,wherein the substrate surface optionally comprises indicia or visiblefeatures.